In the prior art, communication methods such as FDMA (Frequency Division Multiple Access) and OFDMA (Orthogonal Frequency Division Multiple Access) have employed a method of dividing the frequencies allocated to the entire system and allocating different frequencies to adjacent cells, in order to avoid interference between adjacent cells (see for example Japanese Laid-open Patent Publication No. 2004-159345).
FIG. 13 is used to explain the frequency allocation method. As depicted in the figure, in the method, for example three cells (cells #A to #C) are allocated different frequencies (f1 to f3). Near cell boundaries, two terminal apparatuses positioned in different cells use different frequencies, so that there are no problems with interference, and good-quality communication with a base station apparatuses is possible.
FIG. 14 and FIG. 15A to FIG. 15C depict an example of another method of frequency allocation described in the above Japanese Laid-open Patent Publication No. 2004-159345. As depicted in FIG. 13 and similar, a common frequency (f4) is allocated in regions near the base station in each cell, and in regions near boundaries with adjacent cells, frequencies which are different from those of adjacent cells (f1 to f3) are allocated.
In the frequency allocation method in which different frequencies are allocated to adjacent cells (FIG. 13), the frequency utilization efficiency in each cell is “⅓” of the overall frequency band. On the other hand, in the frequency allocation method in which different frequencies are allocated to adjacent cells while using the common frequency (FIG. 14 and elsewhere), the frequency utilization efficiency is “ 2/4=½”. The frequency efficiency of frequency allocation using the common frequency is better than that of frequency allocation not using the common frequency.
On the other hand, cell configuration methods include an “omni cell” and “sector cell” methods. The “omni cell” is a cell using a non directional antenna in the base station apparatus. A “sector cell” is a cell using a directional antenna. FIG. 16 and FIG. 17 depict examples of “omni cell” and “three-sector cell” configurations respectively. In both cases, three frequencies, “f1” to “f3”, are used, and different frequencies are allocated between adjacent cells and adjacent sectors.
Focusing on the cell and sector to which the frequency “f1” is allocated, in the “omni cell” (FIG. 16), radio waves arrive from each of the base station apparatuses 100 of the six adjacent cells, so that there is a high possibility of interference. On the other hand in the “three sector cell” (FIG. 17), the directional antenna is used, so that radio wave is not received from base station apparatus in the diagonal upper-left direction, and to this extent there is less interference compared with the “omni cell” case.
FIG. 18 depicts an example of a “six-sector cell”. In the “six-sector cell”, different frequencies are allocated to adjacent sectors, and the frequency utilization efficiency in each sector is “⅙”.
As explained above, an occurrence of interference of the “sector cell” is less than of the “omni cell”. The method of frequency allocation which utilizes the common frequency (FIG. 14) is a method using the “omni cell”. By applying the frequency allocation method to the “sector cell”, the occurrence of interference can be reduced.
However, the frequency utilization efficiency of the “sector cell” has lower than the “omni cell”. For example, the frequency allocation utilization efficiency of the “omnicell” depicted in FIG. 14 is “½”, but the frequency allocation utilization efficiency of the “six-sector cell” system depicted in FIG. 18 is “⅙”.